A few years ago I wrote something to a female German teacher who I was quite fond of (not in a romantic way), and it included the phrase:

Ich habe Sehnsucht nach dir.

I was shooting for a translation to "Estou com saudade de você." This is an expression in Portuguese (my native language), which roughly translates to "I miss you" in English and it's quite common to use it when talking to friends.

At the time, it didn't occur to me that this expression might have a stronger romantic connotation in German. So, German speakers, did I commit a faux pas writing something inappropriate here? If yes, what would've been a more appropriate expression?

"Sehnsucht nach jmd. haben" is used exclusively in a romantic context in Germany. "Vermissen" is much weaker here. All romantic connotation can also be taken out when we specify what we miss (then we may also use "sehnen").